Re: set_cheapest without checking pathlist

I don't see any inherent reason why we must always assume that
gather_grouping_paths will always result in having at least one entry
in pathlist. If, for example, we've disabled incremental sort and the
cheapest partial path happens to already be sorted, then I don't
believe we'll add any paths. And if that happens then set_cheapest
will error with the message "could not devise a query plan for the
given query". So I propose we add a similar guard to this call point.